This invention relates to an air-to-fuel ratio control system for a vehicle engine and more particularly to an air-to-fuel ratio control system which can make an judgement of inactivity of an exhaust sensor when the vehicle engine is in a specified operating range.
There are heretofore known various feedback or closed loop fuel mixture control systems in which an air-to-fuel ratio detector or exhaust sensor is sensitive to the oxygen content of the exhaust. The fuel mixture control system determines the proper air-to-fuel ratio and constantly monitors the exhaust to verify the accuracy of the air-to-fuel mixture setting. For avoiding improper operation of such an air-to-fuel ratio control system in the case of a breakdown or failure of the air-to-fuel ratio detector or exhaust sensor, it is necessary to detect whether the air-to-fuel ratio detector operates normally or not.
One such air-to-fuel ratio control system being capable of finding a breakdown or failure of the air-to-fuel ratio detector is disclosed in, for example, Japanese patent application No. 59-27,820 filed on Feb. 16, 1984 and laid open to the public on Sept. 6, 1985 under the publication No. 60-173,332. The air-to-fuel ratio control device disclosed in the above mentioned application decides a failure of the air-to-fuel ratio detector when the air-to-fuel ratio detector provides an output representative of a lean mixture under the condition that the quantity of fuel delivered to the airstream is correctively altered not decreasingly but increasingly in open loop fuel control while a vehicle engine is in motion under a specified operating condition.
This failure decision is based on an assumption that the fuel mixture will actually become rich when a fundamental quantity of fuel to be delivered is controlled to alter increasingly in open loop fuel control. However, even when the above condition is satisfied, if an increasing correction value is small, an actual fuel mixture sometimes tends to become lean due to the scattering of the measured quantity of intake air and/or of the quantity of fuel delivered to the airstream by the fuel injector, resulting in an inaccurate failure detection of activity of the air-to-fuel ratio detector. In the case of making such a failure decision of the air-to-fuel ratio detector only when an increasing correction value has become sufficiently large in an attempt at overcoming the above stated inaccurate failure decision, limits of engine operating condition wherein a large value of increasing correction value is attained will become too narrow, resulting in little opportunity to detect a failure of the air-to-fuel ratio detector.